The disclosure of Japanese Patent Application No. 2000-189635 filed on Jun. 23, 2000, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a diagnostic apparatus and method for a fuel vapor purge system for use in an internal combustion engine installed in a motor vehicle, such as, for example, an automobile.
2. Description of Related Art
In the internal combustion engine having the aforementioned fuel vapor purge system, fuel vapor may leak from a canister or a fuel tank into the ambient air if a hole or holes is/are formed in a pipe defining the purge path, or the pipe is disengaged or detached from another component for some reason. In order to detect this situation, it is desirable to automatically detect leakage of fuel vapor from the purge path of the fuel vapor purge system including the canister and the fuel tank.
To meet this requirement, a system for diagnosing the fuel vapor purge system has been proposed in which leakage in the purge path is detected based on a pressure change within the purge path after a negative pressure of the intake system of the engine, which is lower than the atmospheric pressure, is introduced into the purge path and the purge path is then sealed, and also based on a pressure change within the fuel tank due to fuel vapor generated in the tank, which change is measured when the purge path is subjected to the atmospheric pressure and is sealed in this state. When diagnosis of the purge path is effected by detecting a change in the internal pressure of the purge path with time while the purge path is subjected to a negative pressure, it is impossible to determine whether an increase in the pressure within the purge path is caused by the atmospheric pressure entering the purge path through a hole(s) or a crack(s) in a pipe defining the purge path, or the pressure increase is caused by a large amount of fuel vapor generated in the fuel tank. Accordingly, this system is adapted to measure a pressure change before a negative pressure is introduced into the purge path, and also measure a pressure change after the atmospheric pressure is introduced into the purge path.
The aforementioned diagnostic apparatus for the fuel vapor purge system is adapted to measure a change in the fuel tank pressure due to fuel vapor generated in the tank, after a negative pressure is introduced into the purge path for detecting leakage in the purge path. Accordingly, a diagnostic operation to detect leakage in the purge path is performed even when a large amount of fuel vapor is generated within the fuel tank and it is difficult to accurately detect leakage in the purge path. In this case, however, the leakage detection under the negative pressure is an unnecessary step, which results in an increase in time required for diagnosing the fuel vapor purge system.
Furthermore, in the aforementioned fuel vapor purge system in which the fuel tank and the canister are always held in communication with each other, it is necessary to seal the purge path by closing a pressure block valve and a purge control valve so as to measure a pressure change in the fuel tank due to fuel vapor generated in the tank. During the measurement of the tank pressure change, therefore, a purging operation is suspended, in other words, purge cut is effected. If a diagnostic operation to detect leakage in the purge path is performed even when a large amount of fuel vapor is generated in the fuel tank and accurate detection of leakage is difficult, purging is suspended or stopped for an increased period of time, and the fuel vapor purge system may fail to ensure a required amount of fuel vapor to be purged, which should remain in the canister.
It is an object of one aspect of the invention to provide a diagnostic apparatus and method for a fuel vapor purge system, which is able to suppress or avoid an increase in the time required for diagnosing the system by eliminating an unnecessary detecting or determining step(s).
To accomplish the above and/or other objects, one aspect of the invention provides a diagnostic apparatus and method for a fuel vapor purge system wherein fuel vapor generated in a fuel tank is trapped in a chamber (e.g., a canister), and the fuel vapor trapped in the chamber is purged into an intake passage of an internal combustion engine through a purge path that includes the fuel tank. A controller of the diagnostic apparatus measures a first change in an internal pressure of the purge path after creating a pressure difference between the inside and outside of the purge path and sealing the purge path, and measures a second change in the internal pressure that varies with an amount of fuel vapor generated in the fuel tank, which change is measured while the purge path is sealed for a first predetermined period of time after an atmospheric pressure is introduced into the purge path in which the pressure difference was created. The controller then performs leakage diagnosis to determine whether leakage is present in the purge path, based on the first change and the second change in the internal pressure of the purge path. Furthermore, before the measurements of the first and second pressure changes, a third change in the internal pressure that varies with an amount of fuel vapor generated in the fuel tank is measured while the purge path is sealed for a second predetermined period of time after an atmospheric pressure is introduced into the purge path before the pressure difference is created. The leakage diagnosis is inhibited from being performed when a result of measurement of the third change in the internal pressure is greater than a predetermined value, and is allowed to be performed when the result of measurement of the third change is equal to or less than the predetermined value.
According to the aspect of the invention described above, a change in the pressure that varies with an amount of fuel vapor generated in the fuel tank is measured while the purge path is sealed for the second predetermined period of time after the atmospheric pressure is introduced into the purge path before the pressure difference is created. The leakage diagnosis to determine the presence of leakage in the purge system is inhibited if the measurement result is greater than the predetermined value, and is permitted if the measurement result is equal to or less than the predetermined value. Accordingly, when a large amount of fuel vapor is generated in the fuel tank, and it is difficult to accurately detect leakage in the purge path, unnecessary steps of measuring a change in the tank pressure with the pressure difference being provided, and measuring a change in the pressure due to fuel vapor generated in the tank after the creation of the pressure difference, can be advantageously eliminated, resulting in an otherwise possible increase in the time required for accomplishing the diagnosis.